A high frequency plasma generator device used in a plasma chemical vapor deposition apparatus performs forming or etching the semiconductor films from amorphous silicon (hereinafter abbreviated “a-Si”), microcrystal silicon, multicrystal thin films silicon, silicon nitride, etc. for use in solar cells, thin film transistors or the like, and performs self-cleaning for the silicon deposits in the film forming chamber using NF3 gas. The high frequency plasma generator device may employ one of two types of electrodes: a parallel flat plate type electrode or a ladder type electrode.
FIG. 6 illustrates an example of a device employing the parallel flat plate type electrode. Substrate heating support means 6 is installed inside of the plasma chemical vapor deposition apparatus 1 and contacts to the ground electrically. Flat plate electrode 60 is separated, for example, by 20 mm from the opposing substrate heating support means 6. The flat plate electrode 60 is connected to an external high frequency power source 61 via an impedance matching controller 62 and coaxial cable 63. In addition, ground seal 5 prevents the generation of unwanted plasma on the side opposite from the side facing the substrate heating support means 6.
Then, substrate 7 for the a-Si film forming is placed atop the substrate heating support means 6 at a set temperature, for example, of 200° C.; silane gas (SiH4) is introduced via the gas inlet pipe 64 at a flow velocity, for example, of 50 sccm; and a vacuum pump (not shown) connected to vacuum exhaust pipe 65 is operated to adjust the exhaust flow rate, and resultingly, the inside pressure in the plasma chemical vapor deposition apparatus 1 is set, for example, to 100 mTorr. Then, the plasma 66 is generated between substrate 7 and the flat plate electrode 60 by supplying high frequency power from the high frequency power source 61, which is adjusted by the impedance matching controller 62, to create highly efficient plasma generation in the plasma generator device. Inside of plasma 66, the SiH4 breaks down and an a-Si film is formed on the surface of the substrate 7. An adequate thickness of a-Si film may be deposited in this manner over the course of approximately 10 minutes.
An example of a ladder type of high frequency plasma generator device structure is shown in FIG. 7; FIG. 8 shows a diagram from the A-direction in FIG. 7 to further elucidate the structure of the ladder-type electrode 70. The structure of such ladder-type electrodes 70 is described in detail in Japanese Patent Publication Hei 4-236781, etc., while Japanese Patent Publication Hei 11-111622 discloses an evolved form of the ladder-type electrode in which a plurality of parallel electrode rods are arrayed into an electrode group, and where two such groups are arrayed orthogonally to form an electrode mesh.
In the high frequency plasma generator device shown in FIG. 7, substrate heating support means 6 (not shown in FIG. 8)is electrically connected to the ground, which is provided inside of the plasma chemical vapor deposition apparatus, and ladder-type electrode 70 is positioned opposite to the substrate heating support means 6 at a distance, for example, of 20 mm. The ladder-type electrode 70 is electrically connected to an external high frequency power source 61 via impedance matching controller 62 and coaxial cable 63. Ground shield 5 is positioned to prevent unwanted plasma from being generated on the side opposite the surface facing substrate heating support means 6.
Then, substrate 7 for the a-Si film forming is placed atop the substrate heating support means 6 at a set temperature, for example, of 200° C., and silane gas (SiH4) is introduced via the gas inlet pipe at a flow velocity, for example, of 50 sccm. Then a vacuum pump (not shown) connected to vacuum exhaust pipe is operated to adjust the exhaust flow rate, and resultingly, the inside pressure in the plasma chemical vapor deposition apparatus 1 is set, for example, to 100 mTorr. Since the plasma 71 is generated between substrate 7 and the ladder-type electrode 70 by supplying high frequency power to the ladder-type electrode 70, the impedance matching controller 62 can be adjusted to create highly efficient plasma generation in the plasma generator device. Inside of plasma 71, then, the silane (SiH4) breaks down and an a-Si film is formed on the surface of the substrate 7. Thus an adequate thickness of a-Si film may be deposited in this manner over the course of approximately 10 minutes.
Comparing the structural example of FIG. 7 to that of FIG. 6, first, the use of a ladder-type electrode comprised of a plurality of rods having a round cross section rather than a flat plate electrode, allows the silane (SiH4) raw material to flow freely between the rungs to make the supply of the raw material more uniform. Second, the plasma can be generated more uniformly by means of the plurality of power supply points (in this example 4 points), as opposed to one supply point.
Such plasma chemical vapor deposition apparatus include a high frequency plasma generator device, but there is an ongoing demand for devices that can fabricate thin film semiconductors for solar cells, thin film transistors for flat panel displays and the like, in low cost by manufacturing a large surface area (e.g. approximately 1.5×1.2 m) at high speed, and in high quality by lowering the defect density and achieving the high level crystallization rate. Additionally, a demand exists for high speed, large surface area self-cleaning using NF3 gas of the a-Si deposits that accumulate in the chambers where the large surface area film forming takes place, which is the same demand for fabricating thin films.
To meet such demands, very higher frequency power sources (30–800 MHz) have been used for a new plasma generation method. The use of the foregoing very higher frequencies speeds up the film forming rate and produces higher quality. The method is described, for example, in “Mat. Res. Soc. Symp. Proc., Vol. 424, pp-9, 1997. Recently, attention has been focused upon forming microcrystal Si thin films in place of a-Si films using such very higher frequencies to produce high speed, high quality films.
One problem with these very high frequency film forming methods is the difficulty of obtaining uniformity in large surface area film forming. Since the wavelength of these very high frequencies is on the same order as the electrode size, a standing wave is generated at the ends of the electrodes due to reflected waves causing a floating inductance capacitance that affects the voltage distribution, which in turn causes nonuniformity in the plasma distribution due to the interference between the plasma and the high frequency waves. The result is a nonuniform distribution of the film thickness, with the film being thinner in the center. Further, the NF3 plasma used in self-cleaning, being a negatively charged gas (electrons adhere easily), causes the plasma itself to be very unstable, and in addition its distribution is nonuniform due to the gas flow (the plasma being generated on the downstream side) and differences in the distance between electrodes.
In a typical example employing the parallel flat plate electrode that is structurally depicted in FIG. 6, the electrode size exceeds 30 cm×30 cm. Thus, when the frequency exceeds 30 MHz, the above described effects become pronounced and it is difficult to achieve even the minimal uniformity in film thickness of ±10% for manufacturing the semiconductor film.
On the other hand, in a typical example with a ladder-type electrode structured as shown in FIGS. 7, 8, the pronounced standing wave that forms from the single point of power supply is characteristically reduced by using four power supply points. However, in this case as well, it becomes difficult to form uniform films when the electrode size exceeds 30 cm or when the frequency exceeds 80 MHz.
The above-described problems have drawn the attention of academic societies, and at this point, as described in Mat. Res. Soc. Symp. Proc., Vol. 377, pp. 27, 1995, for example, proposals were made to connect a no-loss reactance (coil) to the side opposite the power supply side of the parallel flat plate electrode. This changes the reflected wave conditions for the standing wave at the end of the electrode and generates a relatively flat distribution of standing wave waveforms, for example, a sine wave peak over the electrode that thereby decreases the voltage distribution from the electrode. However, this method does not fundamentally eliminate standing waves, but merely generates the flat part of a sine wave over the electrode, which produces a uniform area which is approximately ⅛ of that wavelength. In theory, further uniformization beyond that range would not be possible.
Other proposals for technology to generate uniform plasma over a large surface area is disclosed in Japanese Patent publications 2000-3878, 2000-58465, 2000-323297, 2001-7028, etc., but in these, the maximum electrode size was about 80 cm×80 cm. They cannot accommodate the large surface areas of 1.5 m×1.2 m that are targeted by the present invention. Thus, the conventional technology for plasma chemical vapor deposition apparatus that perform very high frequency plasma generation could not generate plasma to produce a uniform film upon very large substrates that exceed 1 m×1 m in size.
Technology similar to that used in the present invention, that is technology that uses two different discharge electrodes supplied two different high frequency waves is described in detail in, M. Noisan, J. Pelletier, ed., “Microwave Excited Plasmas,” Technology, 4, second impression, pp. 401, Elsevier Science B. V., 1999.
However the objective of that technology was to control the volume of active ions and other inflow as well as the irradiated energy by means of using the one high frequency wave for plasma generation and the other to control the bias voltage on the surface of the dielectric substrate. That objective differs completely from the objective of the present invention of producing uniform plasma generation over an exceeding large substrate of 1 m×1 m.
The present invention was developed to resolve the aforementioned problems and has as its objective, the provision of a uniformization method and device for high frequency plasma over large surface areas, by employing a plasma chemical vapor deposition apparatus that generates uniform plasma over a large surface area using a very high frequency (VHF) plasma chemical vapor deposition apparatus.